A conventional gearbox comprises a main shaft, a counter shaft, and a plurality of gearwheel pairs each comprising a primary gearwheel fixedly arranged on the counter shaft and a secondary gearwheel rotatably arranged on the main shaft. The primary gearwheel and the secondary gearwheel of each gearwheel pair are in constant meshing engagement with each other. A synchronizing unit is used to synchronize the speed of the secondary gear wheels of each gearwheel pairs with the speed of the main shaft and to lock the secondary gear wheel on the main shaft when a synchronous speed has been reached. Usually, a synchronizing unit comprises a conically shaped friction surface connected to the secondary gear wheel and a conically shaped friction surfaces connected to the main shaft. When the conically friction surfaces are engaged, the secondary gear wheel and the main shaft achieve a synchronous speed in a fast manner. However, the friction surfaces of the synchronizing units are subjected to heavy loads resulting in a large wear.
An alternative to use synchronizing units is to use a transmission brake. A transmission brake is normally designed as a multi disc brake. During an upshift process, the transmission brake is used to provide a braking torque on the counter shaft in order to reduce the speed of a secondary gear wheel to be engaged to the speed of the main shaft. As soon as the secondary gearwheel has reached a synchronous speed with the main shaft, the secondary gear wheel is rotationally locked to the main shaft by a locking unit. During a downshift process, the engine is activated such that it provides an acceleration of the counter shaft until a secondary gear wheel to be engaged reaches a synchronous speed with the mains shaft. As soon as the secondary gearwheel has reached a synchronous speed with the main shaft, the secondary gear wheel is rotationally locked to the main shaft by a locking unit.
A conventional transmission brake provides a fixed braking torque on the counter shaft. A gearbox comprising a split gear has a counter shaft with a varying moment of inertia depending on the split gear engaged. It is difficult to dimension a transmission brake providing a fixed braking torque on the counter shaft for such a gearbox. The transmission brake may be dimensioned to the highest moment of inertia of the counter shaft. In this case, the counter shaft receives a very fast retardation, which makes it difficult to disengage the braking torque at the right time with a high accuracy when it has a small moment of inertia. Alternatively, the transmission brake is dimensioned for the lowest moment of inertia. In this case, the counter shaft receives a slow retardation resulting in a long gearshift process when it has a large moment of inertia.
US 2008/0116030 shows a gear brake device for a multi-speed manual transmission. A first gear brake is used during an upshift process. At least one additional auxiliary brake is provided, which acts on an input gear shaft or on a power take off shaft. The auxiliary brake is designed so that it can perform auxiliary functions alone or together with the first gear brake such as a PTO braking function or a hill hold function.